It is known that a plasma protein, which is a plasma colloid, plays an important role in maintaining the amount of blood in blood vessels of a living body. Therefore, up to now, techniques for fluid replacement with a plasma expander having a colloid osmotic pressure approximately equal to that of blood of a living body have been applied for recovering hemorrhagic shock of a patient. However, in the case where a patient bleeds in an amount of 30% or more of the circulating blood, oxygen is insufficiently provided to peripheral tissues, so that the patient requires administration of an oxygen carrier in addition to a plasma expander.
Conventionally, natural blood containing natural erythrocytes or an erythrocyte concentrated solution has been used as such an oxygen carrier. However, in order to avoid blood clot caused by antigen-antibody reaction, it is required that the blood type of a donor matches that of a receptor and cross-compatibility is performed when in use. In addition, the effective storage period of such natural blood or erythrocyte concentrated solution is as short as 3 weeks (4° C.). Frozen blood capable of long-term storage has problems such as high cost and occurrence of hemolysis by osmotic pressure shock when using it. Moreover, occurrence of infectious diseases such as hepatitis and acquired immunodeficiency syndrome was also a concern.
In order to solve the aforementioned problems, the following studies have already been disclosed: a study on an artificial oxygen carrier composed of a fluorocarbon emulsion (JP 60-33367); a study on a blood substitute for extending retention time of hemoglobin (such as human hemoglobin, bovine hemoglobin, and recombinant hemoglobin, hereinafter abbreviated to Hb), which is a heme protein having functions of adsorbing and desorbing oxygen reversibly and of dissociation, in circulating blood by binding polyethylene glycol (hereinafter abbreviated to PEG) or the like to hemoglobin for modification (JP 2-6337); or the like.
However, the oxygen carrying ability of the artificial oxygen carrier composed of a fluorocarbon emulsion depends on a physical oxygen dissolving phenomenon in fluorocarbon. Therefore, the oxygen carrying ability is insufficient. A blood substituent in which Hb binds to PEG or the like also has a problem of insufficient hemoglobin retention time.
The development of an oxygen infusion using Hb such as intramolecular crosslinking Hb, water-soluble polymer bound Hb, or intermolecular crosslinking polymerization Hb has been advanced. However, the oxygen infusion has been found to have various side effects caused by the non-cell structure.
Reasons that Hb is naturally encapsulated in erythrocyte membrane are shown in the following items:                1) to enable controlling a colloid osmotic pressure by being encapsulated by erythrocyte membrane because the colloid osmotic pressure of an Hb solution exceeds a physiological value when the Hb concentration is 5% or more;        2) to suppress the side effects by encapsulating hemoglobin having high physiological activity in membrane;        3) to retain various phosphate compounds or a glycolysis/reductase system for maintaining Hb functions such as oxygen affinity and stability of an oxygen complex in the same system; and        4) to have an advantage that a hemocyte dispersed system, which is a non-Newtonian fluid, shows a physiological function by distinctive flow form when circulating it in the body (particularly peripheral blood vessels).        
Considering such primary roles of erythrocyte structure, it is clear that a dispersion system containing particles formed by encapsulating Hb in a capsule is suitable as the oxygen infusion.
On the other hand, it has been discovered that a phospholipid that is a biological component forms a vesicle structure independently, and many institutes have made studies on a hemoglobin vesicle since Djordjevich and Miller started researches on a hemoglobin vesicle using a liposome composed of phospholipid/cholesterol/fatty acid. The use of the hemoglobin vesicle has advantages in that 1) natural hemoglobin can be used without treatment, 2) the side effects caused by hemoglobin can be suppressed, 3) the viscosity, colloid osmotic pressure, or oxygen affinity can be adjusted to an arbitrary value, and 4) the retention time of blood in the body can be extended.
Various researches have been done for a liposome-encapsulated hemoglobin in which hemoglobin is encapsulated inside the layer of a liposome composed of lipid bilayer membrane (JP 52-151718, JP 58-183625, and JP 61-37735). However, a suspension of such a liposome-encapsulated hemoglobin has an extremely low colloid osmotic pressure, so that the effect of maintaining or increasing the circulating blood amount cannot be expected even when the suspension is administered to the blood vessel of a living body.
As alternatives to overcome the aforementioned defects, there has been disclosed artificial blood having both an oxygen carrying ability and a plasma expanding effect, which is formed by suspending a liposome-encapsulated hemoglobin in an aqueous solution of a plasma expander (U.S. Pat. No. 4,133,874 and JP 4-5242).
However, for the artificial blood disclosed in U.S. Pat. No. 4,133,874, the liposome-encapsulated hemoglobin tends to aggregate in the aqueous solution of the plasma expander. Therefore, the viscosity of the suspension becomes high, so that the suspension has difficulty in intravenous administration and there is a danger of the liposome aggregates causing embolization in a blood vessel.
In order to suppress an interaction between a hemoglobin vesicle and a protein in blood, the artificial blood disclosed in JP 4-5242 suppresses an interaction between a hemoglobin vesicle and a plasma expander by using an oxygen carrier in which PEG is bound to the hemoglobin vesicle surface (i.e., phospholipid) (JP 2-149512 or JP 3-218309).
However, it is known that when a water-soluble polymer such as PEG (synonymous with polyoxyethylene) is bound to a diacyl phospholipid such as phosphatidylethanolamine that is a major cell membrane component of a general living organism, the diacyl phospholipid bound to the water-soluble polymer is desorbed from a phospholipid bilayer membrane vesicle (J. R. Silvius and M. J. Zuckermann. Biochemistry, 32, 3153, 1993 and JP 2001-64383, paragraph 0004).
A pharmaceutical composition containing the aforementioned artificial oxygen carrier had the following problems. The administration of the composition to a living body without providing a colloid osmotic pressure causes a shock symptom. Moreover, in the case where the electrolyte balance differs from a physiological electrolyte balance, hemolysis or angiitis is caused due to angialgia when the pharmaceutical composition is administered to the living body or a difference between the crystalloid osmotic pressures. In addition, when the pH of the artificial oxygen infusion is higher than physiological pH, alkalosis is easily caused, whereas when the pH of the artificial oxygen infusion is lower than physiological pH, acidosis is easily caused.